A conventional hairpin, as shown in FIG. 1, comprises a curved plastic plate combined with a curved metal plate B by means of two projecting posts C set on the plastic plate inserted and fused in two holes D in the metal plate B. In combining process heat fusion of the post C must be performed by a heat gun, which wastes time and labor and is rather inconvenient. Besides, the fusing time has to be controlled to be correct, otherwise combination of the plastic plate with the metal plate would not be tight, and the metal plate may easily loosen off the plastic plate in the result.